This invention relates to random access to a compressed video data stream (bit stream), and in particular to methodology and a system (or apparatus) for furnishing and enabling low-latency random access to such a stream.
In both the systemic and methodologic realms, the invention involves, among other things, both the preparing of a conventional video data stream for low-latency, and quickly achieved high-quality (resolution) random access, and the ultimate reception and utilization (i.e. viewing) of that prepared data stream. For the purpose of illustration herein, a preferred and best-mode embodiment of, and manner of implementing and practicing, the invention are disclosed and illustrated in the context of television channel surfing-a representative setting wherein the invention has been found to offer special utility. This context is aptly representative generally of the invention's useful applicability in the areas of video broadcasting and video streaming.
By way of general background regarding conventional understanding, due to temporal prediction, a video decoder cannot begin decoding a video data stream at a frame that is predicted from previous frames. Many applications require a user to be able enter a bit stream at any time, such as during the activity known as channel surfing between broadcast streams, and during reverse, or fast-forward, “trick” video modes. A typical technique for providing random access to a compressed bit stream involves the insertion of so-called I-frames, or intra-frames. Such I-frames are pictured and employed herein, and are also referred to as video marker frames. I-frames are typically inserted into a so-called Group Of Pictures (GOP) structure, and are coded without any prediction from other frames. They are inserted at an interval, or rate, (R). A decoder is able to access an associated video data stream at any such inserted I-frame. The latency involved in random access in such a GOP is inversely proportional to R, while the compression performance decreases as R increases. As an example, I-frames are typically inserted periodically in MPEG-2 format to provide random access.
Another concept in the prior art involves the so-called concept of “dirty random access”, regarding which an access point does not begin with a clean I-frame. Rather, in this approach, a decoder begins decoding and displaying predicted frames without having access to a complete reference frame. Blocks of the frames are intra-coded so that, after some time, a clean picture is attained.
Other approaches to dealing with random access latency have been proposed in the prior art and are generally familiar to those generally skilled in the art.
The present invention departs from prior art approaches by proposing the creation, from an “engaged” source video data stream, of a compressed and either slightly time-offset (preferable), or time-synchronized, pair of divided video data streams (called downstream-deliverable streams) that are drawn directly from the source stream, and that are characterized by possessing respective, different access latencies and resolutions. In particular, and according to a preferred manner of practicing and implementing the invention, such a source data stream is split into two data streams which are slightly time-offset with respect to one another, with one of these streams being specifically characterized with a relatively low access latency (the stream which is slightly time-delayed relative to the other stream) and a relatively low image resolution, and with the other stream being characterized by a larger access latency, and a significantly larger, or greater, image resolution.
These two streams are preferably multiplexed and transmitted so as to be received at a receiving site where they can, in terms of certain fundamental steps practiced by a portion of the present invention, be (a) sought, (b) monitored, (c) and selected in a manner, which uniquely introduces a definitive low-latency access, followed by a rapidly achieved, high-resolution image display.
These, and various other features and advantages, which are offered and attained by the system, apparatus and methodology of the present invention will be more fully appreciated in light of the detailed description which shortly follows, when read in conjunction with the accompanying drawings.